1. Field of the Invention
The present invention relates to a liquid crystal display device, in which a display medium including liquid crystal regions surrounded by a polymeric wall and sandwiched between a pair of opposing substrates, applicable to a plane display device such as a personal computer, a liquid crystal television set, a portable display including a film substrate, and a display apparatus in which a display portion and an input portion are integrated for adopting a pen entry operation.
2. Description of the Related Art
Conventional liquid crystal display devices utilize a variety of display modes as follows: As liquid crystal display devices utilizing an electro-optic effect, a twisted nematic (TN) liquid crystal display device and a super-twisted nematic (STN) liquid crystal display device using nematic liquid crystal have been put to practical use (Japanese Laid-Open Patent Publication No. 59-119320). Further, as an STN liquid crystal display device achieving a brighter display, a reflective type liquid crystal display device including one polarizing plate is proposed (Japanese Laid-Open Patent Publication Nos. 4-97121 and 4-289818).
As liquid crystal display devices utilizing light scattering of liquid crystal without using a polarizing plate, those utilizing a dynamic scattering (DS) effect and a phase change (PC) effect have been proposed.
Recently, as a liquid crystal display device requiring neither a polarizing plate nor an alignment treatment, a display device utilizing the birefringence of liquid crystal so as to electrically control the transparent state and the opaque state has been proposed. In such a liquid crystal display device, the refractive index of the liquid crystal molecule with respect to ordinary light is basically matched with the refractive index of a polymeric material supporting the liquid crystal. As a result, the transparent state is displayed when a voltage is applied to uniformly orient the liquid crystal molecules, and the opaque state is displayed under application of no voltage because of light scattering caused by the turbulence of the liquid crystal molecules. It is disclosed that such a liquid crystal display device is produced by using a mixture of a photosensitive or thermosetting material and liquid crystal, from which the photosensitive or thermosetting material alone is deposited by polymerization, thereby forming a liquid crystal droplet surrounded by the polymeric material (resin) (Japanese National Publication No. 61-502128).
As another type of a liquid crystal display device, a non-scattering type liquid crystal device having viewing angle characteristics improved by using a polarizing plate, is also proposed (Japanese Laid-Open Patent Publication No. 5-27242). This liquid crystal display device is produced by causing phase-separation in a mixture of liquid crystal and a photopolymerizable material to obtain a complex material for a liquid crystal domain and a polymeric wall. The thus formed polymeric wall causes random alignment of the resultant liquid crystal domains, and hence, the liquid crystal molecules in the respective liquid crystal domains rise in different directions. As a result, the apparent refractive index is constant when seen from any direction, and thus, the viewing angle characteristics at halftone are improved.
As a similar liquid crystal display device, the Applicant has proposed the following device (Japanese Patent Application No. 4-286487): In producing this liquid crystal display device, light control means such as a photomask is used in light irradiation to cause photopolymerization in a mixture of liquid crystal, a photopolymerizable material and the like, thereby forming liquid crystal domains omnidirectionally, i.e., radially, oriented in a pixel region. Accordingly, the liquid crystal molecules are moved in such a manner that an umbrella is opened or closed by controlling a voltage to be applied. This liquid crystal display device has further improved viewing angle characteristics.
In the liquid crystal region in the above-mentioned conventional liquid crystal display device, the liquid crystal is aligned by using an alignment regulating force on a substrate. Since the polymeric wall is an isotropic phase, however, its color is different from that of the liquid crystal region, resulting in decreasing the brightness in displaying white. This problem is particularly severe in the reflective type liquid crystal display device for the following reason: When the reflective type liquid crystal display device includes a non-pixel portion formed from a material of the isotropic phase, the isotropic phase between the polarizing plates is in the similar state as that for displaying black. Therefore, the display obtained by such a reflective type liquid crystal display device is generally dark.
Japanese Laid-Open Patent Publication No. 4-323616 discloses a method for producing a liquid crystal display device in which partitions are previously formed on a substrate to be used for manufacturing a cell and liquid crystal is injected into the cell. In this production method, however, the alignment direction between the partition and the liquid crystal is different from the alignment direction by an alignment film on the substrate. Therefore, the alignment direction of the liquid crystal is disordered in the vicinity of the partition, resulting in decreasing the contrast. Moreover, since the partitions are formed on the substrate and then a counter substrate is attached to the substrate, there is no material between the substrates to attach them to each other, resulting in decreasing the physical strength of the cell. Furthermore, since an alignment treatment is conducted on the alignment film on the substrate before forming the partition on the substrate by photolithography or the like, the alignment regulating force on the substrate is weaken, and hence it is impossible to obtain excellent display characteristics. In addition, it is difficult to adjust the height (in the direction vertical to the surfaces of the substrates) of the partitions on the substrate, and a spacer is additionally used to obtain the desired thickness of the cell. Therefore, the thickness of the cell is varied depending upon whether or not the spacer is positioned on the partition, resulting in difficulty in controlling the thickness of the cell accurately.
In such a conventional liquid crystal device, the thickness of a liquid crystal layer, i.e., the so-called cell thickness, is varied with ease by an external pressure. Therefore, when the liquid crystal display device adopts a pen entry operation, the pen entry causes local display irregularity. Therefore, in order to avoid this irregularity, it is necessary to provide a protection film (a protection substrate) or the like over the liquid crystal display device. The use of the protection film or the like leads to a larger distance between the display portion and the pen entry portion, and this distance causes a parallax, causing a difficulty in the operation.
In a liquid crystal display device having improved wide viewing angle characteristics, a sufficient duty ratio cannot be obtained because of the lack of the sharpness in the electro-optical characteristics. Therefore, it is necessary to use an expensive thin film transistor (TFT), resulting in a high production cost.